A person close to Prince William says the heir to the throne refuses to be "hounded" by paparazzi.

The British royal and his wife Duchess of Cambridge Catherine Middleton are expecting their first child together. Kate has been admitted to hospital with acute morning sickness and her admission meant the couple had to make public the news of the pregnancy, which they intended to keep to themselves until Christmas Day.

Sources said the Duchess of Cambridge was bearing up well, but severe vomiting may last for much of her pregnancy after she developed the rare illness hyperemesis gravidarum.

Her illness has led to speculation she may be expecting two babies, with experts saying the condition was slightly more common with twins.

Since news of the pregnancy, the world's media have gathered outside the hospital where Kate is being treated. However, William will reportedly not allow a repeat of what happened with his mother, Princess Diana of Wales.

The princess died in a car accident in 1997 and was shadowed by the press throughout her adult life.

But in September, French magazine Closer printed topless pictures of Catherine as she and Prince William were enjoying a holiday in Provence, sparking international scandal and a lawsuit.

Prince William will reportedly not allow Catherine or his unborn child to undergo similar media pressure.

"William has always had a distrust, bordering on outright loathing, for the press," a source told Radar Online.

"It all stems from his childhood - watching the way his mother was built up and then torn down by the media and ultimately, he strongly believes to this day, hounded to her death by overzealous paparazzi.

"He has vowed to do everything in his power to protect Kate from the media frenzy surrounding her pregnancy and Heaven help anyone who crosses the line," the person said.

"At William's behest the Queen instructed Palace bigwigs to individually call all of the chiefs at the British newspapers immediately following the pregnancy announcement and warn them, in no uncertain terms, that if any lines are crossed, any intrusive paparazzi shots are purchased and published, or if Kate feels hounded in anyway there will be hell to pay.

"William was deeply hurt and angered by the topless photo scandal Kate was forced to endure over the summer, and has made it crystal clear that, make no mistake, he will not allow his wife to be subjected to such blatant intrusion and humiliation ever again."